1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof and particularly relates to a semiconductor device using an oxide semiconductor. The present invention also relates to an electronic appliance equipped with the semiconductor device.
2. Description of the Related Art
Flat panel displays (FPD), typified by liquid crystal displays (LCD) and EL displays, have attracted attention as the display device replacing conventional CRTs. The development of large screen liquid crystal television mounted with an active matrix-driven large scale liquid crystal panel is particularly an important challenge which liquid crystal panel makers should focus on. In addition, large screen EL television is also being developed.
In the conventional liquid crystal device or electroluminescence display device (hereinafter referred to as a light emitting display device or an EL display device), a thin film transistor (hereinafter referred to as TFT) is used, which uses crystalline silicon or amorphous silicon as a semiconductor element driving each pixel.
A TFT using a crystalline silicon film has a higher mobility by two digits or more compared to a TFT using an amorphous silicon film, and has potential for high speed operation when it is used for a scanning line driver circuit for selecting a pixel of a light emitting display device, a signal line driver circuit for sending video signals to a selected pixel, or the like. However, using crystalline silicon for a semiconductor film complicates manufacturing steps because of crystallization of the semiconductor film compared to using amorphous silicon for the semiconductor film; therefore, there are drawbacks of yield decrease by that much and increase in cost. Further, a heating temperature for the crystallization is 550° C. or higher, and it is difficult to use a substrate made of a resin with low melting point, a plastic substrate, or the like.
On the other hand, the TFT using amorphous silicon for a semiconductor film can be manufactured at low cost, since it is not heated at a high temperature and a resin substrate or a plastic substrate can be used. However, a mobility of only around 0.2 to 1.0 cm2/V·s at most can be obtained with a TFT of which a channel forming region is formed with a semiconductor film formed of amorphous silicon, and it also has high power consumption.
A plasma CVD method is commonly used when an amorphous silicon film is formed over a substrate. Film formation by a plasma CVD method requires heating under high vacuum, and damage to a plastic substrate or an organic resin film over a substrate is a concern. In addition to the concern in forming the amorphous silicon film by a plasma CVD method, there is also a concern in forming the film by a sputtering method which is that a thin insulating film might be formed over a surface of an amorphous silicon film when the amorphous silicon film is exposed to atmospheric air.
As a material to replace a semiconductor made of such silicon, forming a TFT using an oxide semiconductor such as zinc oxide for a channel forming region has been reported in recent years (for example, refer to Patent Document 1: Japanese Patent Laid-Open No. 2000-150900, and Non-Patent document 1: Elvira M. C. Fortunato, et al. Applied Physics Letters, Vol. 85, No. 13, P 2541 (2004)). Since the oxide semiconductor has mobility equal to or higher than that of a TFT formed with a semiconductor including amorphous silicon, further characteristic improvement is demanded.